Project Summary/Abstract My overarching goal is to understand the mechanism by which prominent members of the human microbiota modulate host aging and age-associated health decline. The human intestinal tract is inhabited by trillions of microorganisms, collectively referred to as the microbiota, which contributes to gastrointestinal health and systemic immunity. While metagenomic sequencing has revealed age-associated compositional changes in the gut microbiota, how individual bacterial species of the human microbiota functionally contribute to host aging physiology remain largely unexplored. Recent studies have uncovered a small number of gut microbiota-derived molecules that can bind to cell-surface and nuclear receptors in host cells and extend lifespan in model organisms, unraveling the potential of microbiota-dependent molecules to impact human health. However, the human microbiota produces molecules that are vast in numbers and chemically diverse, posing a tremendous challenge for the field of microbiome science to systematically and accurately identify them. To overcome this challenge, I built a comprehensive chemical reference library and a mass spectrometry-based metabolomics pipeline, which enables rapid and high-throughput identification of over 1000+ metabolites in diverse host samples. My metabolomics profiling of 100+ individual prominent human gut species and of gnotobiotic mice colonized with individual model gut microbes uncovered a panel of high abundance, conserved gut microbe-derived molecules. These candidates are tantalizing candidates for modulating host physiology. Spermidine, one candidate from the polyamine pathway, has been shown to extend healthspan and lifespan in mammals. However, the role of gut microbe-dependent polyamine biosynthetic pathway in modulating host aging has not been investigated. Furthermore, the interactions between the remaining candidates and conserved longevity pathways in the host are largely unknown. The goal of my proposal is to investigate the molecular mechanisms by which microbiota-derived molecules regulate mammalian host health and longevity. Specifically, I hypothesize that a subset of these candidate molecules impact aspects of host physiology via regulating gastrointestinal health and systemic aging. My experiment will use genetic manipulations of model gut microbes such as Bt in the gnotobiotic mouse experimental system to study the impact of microbiota-derived molecules on host aging biology. Using a combination of mass spectrometry, metabolomics, and microbial genetics, this project will i) investigate the role of gut microbiota-dependent polyamine biosynthesis in regulating age-associated decline in host gastrointestinal function, and ii) Identify high-abundance, gut microbiota-derived small molecules that impact host intestinal health and organismal longevity. This study will provide new insights into the mechanistic relationships between gut microbiota, small bioactive molecules, gastrointestinal health, and aging.